The discontinuous mode flyback converter is a commonly used topology for low power AC/DC adapters used with consumer electronic devices. Such consumer electronics devices tend to be cost sensitive, yet also be able to meet increasingly stringent energy efficiency requirements. It is not uncommon for such AC/DC adapters to remain plugged in to a power outlet, even when the electronic device is not connected to it. Meeting energy efficiency requirements in this mode of operation necessitates a very low power consumption mode of operation, sometimes known as a “standby mode.” Other operating modes can include fixed frequency mode, valley switching mode, or boundary/variable frequency mode.
In any of these operating modes, the converter will experience both variable and fixed power losses. Variable power losses increase or decrease with output power, and include conduction losses related to various circuit resistances as well as transformer core losses. Fixed power losses remain substantially constant regardless of output power delivered to the load. Such fixed power losses include gate drive power losses, various capacitive switching losses, and other phantom loads such as internal bias power, preloads, etc. Although these fixed power losses are negligible at maximum power, they become a significant percentage of losses under low load conditions. In boundary/variable frequency mode, frequency increases with decreasing load, which can cause some of the “fixed” losses, such as switching losses, to increase with decreasing output power. To minimize the impact of these, burst mode operation (as described in greater detail below) may be used under light load conditions such as the standby mode.
Other design considerations and/or operating constraints may also require modifications to conventional flyback converter designs. For example, isolation requirements between the input and output of the flyback converter may, in some embodiments, be addressed by providing primary side regulation. With primary side regulation, all of the sensing and control necessary for control of the flyback converter is performed on the input side (as described in greater detail below). When using primary side regulation, ensuring that the output voltage remains sufficiently regulated can become difficult, particularly in low load and/or burst mode operating conditions. Thus, what is needed in the art and disclosed herein are improvements to primary side regulated flyback converters operating in the burst mode.